The present invention relates to a mobile communications device, a communications system, and a communications method for use in mobile communication, and more particularly, to a mobile communications device, a communications system, and a communications method which utilize the spread-spectrum communication technique.
In a mobile communications system utilizing a commonly-employed spread-spectrum communications system in which communication is established between a cell station and a mobile station, a system of the cell station fixes a transmission chip rate fc of the communication, and a transmission symbol rate fv is determined and fixed each time a call involving an individual mobile station is established. More specifically, a spreading rate N (=despreading rate Nr) of spread-spectrum communication is fixed each time a call involving a mobile station is established. Here, the spreading rate N (=despreading rate Nr) corresponds to a ratio of a chipping rate to a symbol rate.
In the common spread-spectrum communications system, communication between a cell station and a mobile station is synchronized and maintained, by means of the mobile station fixing the chipping rate fc for spreading/despreading.
In order to address a problem of a loss of synchronism of chips between a cell station and a mobile station stemming from high-speed movement of the mobile station, as well as to effect RAKE receiving for improving reception sensitivity, data to be used for defining the relationship between a phase and a correlation output over a spreading code period are prepared in the form of a delay profile. The delay profile corresponds to a relationship between the phase "psgr" and a correlation output P("psgr") determined by means of measurement of a correlation output P over all phases within the spreading code period.
FIG. 10 is a plot showing the relationship between a phase "psgr" and a correlation output P("psgr"). As shown in FIG. 10, from the delay profile a plurality of phases "psgr"i for which a correlation output P("psgr"i) is sufficiently greater than noise are determined as a receiving paths. At the time of normal demodulation, the initial phase of a despreading code is synchronized with the phase "psgr"i, thus effective despreading a received signal. Here, the phase "THgr" of the largest correlation output P (xcexa80="THgr") is particularly called a xe2x80x9cprincipal wave.xe2x80x9d
As mentioned above, in the common spread-spectrum communications system, a mobile station periodically prepares a delay profile, thus detecting and updating the phase xcexa8i of a despreading operation to be performed at the time of a normal demodulation operation.
A correlation output P is determined by means of setting into a parallel correlator a despreading code (=a spreading code) C having the characteristic of a pseudorandom number, inputting a received BB signal into a matched filter, where the signal is despread, and detecting a resultant correlation output.
As mentioned above, in the common spread-spectrum communications system, there is prepared a delay profile for defining the relationship between a phase and a correlation output over the entire a spreading code period, to thereby maintain synchronization between a mobile station and a cell station; that is, to prevent loss of synchronism between chips, which would otherwise be caused by a high-speed movement of the mobile station. Preparation of the delay profile (see FIG. 10) for preventing loss of synchronism between chips, which would otherwise be caused by high-speed travel of the mobile station, is performed at a period Td shorter than a period T during which loss of synchronism between chips arises.
FIG. 11 is a plot showing the relationship between the correlation output P and a phase T. As shown in FIG. 11, in order to prevent a failure to detect a path, a phase-shift unit Ts to be used for determining a correlation output P for preparing a delay profile must be narrower than a phase width Tp in which a correlation is output.
In the common spread-spectrum communications system, a transmitter uses a code C having the characteristic of a pseudorandom number as a spreading code, whereas a receiver despreads a received code while the spreading code C transmitted from the transmitter is used as a despreading code.
FIGS. 12A and 12B are plots showing the relationship between the correlation output P and the phase "THgr". FIG. 12A is a discrete model case and shows the relationship between a correlation output P and a phase "THgr", and FIG. 12B is a continuous model case and shows the relationship between a correlation output P and a phase "THgr".
For the sake of simplicity, there will now be described a discrete model case, in which chips are in phase with each other and the width of one step phase ts of a sample to be correlated corresponds to one chip bit (or a chipping period Tc). Here, assume that the phase width Tp of the discrete model corresponds to one chip bit (or the chipping period Tc) and that a chip rate is taken as Fc.
FIG. 13 is an illustration showing the relationship between a despreading code C and a spreading code of a received signal in a case where chips are in phase with each other. As shown in FIG. 13, in the conventional spread-spectrum communications system, in a case where the despreading code C is synchronized with a received signal, the chance of a match arising between bit data assumes a value of 100%, and the chance of a disparity arising between bit data assumes a value of 0%. An expected value of a xe2x80x9cmatch/disparityxe2x80x9d for each bit assumes a value of 1 (i.e., 100%).
FIG. 14 shows the relationship between the despreading code C and the spreading code of the received signal in a case here chips are out of phase with each other by one bit. As shown in FIG. 14, in a case where the despreading code C is out of phase with the spreading code of a received signal; for example, where the despreading code C is out of phase with the spreading code of a received signal by only one bit, the chance of a match arising between transmitted data and received data is xc2xd (50%) and the chance of a discrepancy arising between transmitted data and received data is xc2xd (50%), because the despreading code C has the characteristic of a pseudorandom number. Accordingly, an expected value of a xe2x80x9cmatch/disparityxe2x80x9d for each bit assumes a value of 0 (i.e., 0%).
However, as shown in FIG. 11, in the common spread-spectrum communications system, the number of times (N) detection and calculation of a correlation output required for preparing a single delay profile is performed assumes a theoretical minimal value of N/2 (times) and actually assumes a value of 4N (times), with respect to the number of bits (N) of the spreading code C. Consequently, a CPU (central processing unit) or a DSP (digital signal processor) of the mobile station requires a much longer processing time and a larger processing load is imposed thereon. Further, in order to store the resultant detected and calculated correlation output P("psgr"),memory requires larger storage capacity, thereby rendering the spread-spectrum communications system costly.
Longer processing time required for and a larger processing load imposed on a CPU or a DSP and an increase in the storage capacity of memory result in an increase in the power to be dissipated by a mobile station, thus hindering lengthening of a communication time or a call-await time.
In the common spread-spectrum communications system, in principle, only the mobile station can effect a countermeasure against lack of synchronism between chips, thus imposing difficulty in effecting an effective countermeasure against lack of synchronism between chips.
The present invention has been conceived in light of the drawbacks of the background art and is aimed at providing a mobile communications system, a communications system, and a communications method which effectively and readily enable selection of processing capability and resources of a mobile station and less-costly lengthening of a communication time or a call-await time.
To solve the drawbacks, the present invention provides a mobile communications device having control means for controlling communication and communications means for establishing data communication with respect to a cell station, wherein the communications means comprises: chip clock signal generation means for generating a chip clock signal fc; an antenna radio circuit for receiving and transmitting information; spreading code generation means for generating a spreading code on the basis of the chip clock signal fc output from the chip clock signal generation means; despreading means for despreading a signal s received by the antenna radio circuit; correlation output means for extracting a correlation output P from the signal despread by the despreading means; and spreading means for spreading a transmission signal on the basis of the spreading code.
Preferably, the mobile communications device further comprises computation means for preparing a delay profile on the basis of the correlation output P; a user interface for enabling entry of data or indication of information; storage means for storing data or a program; power level detection means for detecting the level of power remaining in a power source, such as a battery; or speed detection means for detecting the traveling speed of the mobile communications device.
Preferably, the control means controls the chip clock signal generation means such that the chip clock signal fc is determined on the basis of a signal entered by way of the user interface or such that the chip clock signal fc is determined on the basis of the data stored in the storage means. Preferably, the control means controls the chip clock signal generation means such that the chip clock signal fc is determined on the basis of a target value W of the power consumed by the mobile communications device. The target power consumption value W may be determined on the basis of a signal entered by way of the user interface or the data stored in the storage means.
Preferably, the control means determines the target power consumption value W according to Equation (W=E/t) on the basis of a power level E detected by the power level detection means and a predetermined time xe2x80x9ct.xe2x80x9d Further, preferably, the control means determines the predetermined time xe2x80x9ctxe2x80x9d in accordance with a mode selection signal for selecting either a long-hour operation mode or a short-hour operation mode. Preferably, the control means sets the target power consumption value W to a target power consumption value W0 employed during a call-await time. Preferably, the control means sets the target power consumption value W to a target power consumption value W0 employed during a call-await time and the predetermined time xe2x80x9ctxe2x80x9d to a target call-await time. More preferably, the control means sets the target power consumption value W to a target power consumption value W1 employed during communication. Preferably, the control means sets the target power consumption value W to a target power consumption value W1 employed during communication and the predetermined time xe2x80x9ctxe2x80x9d to a target available time.
Preferably, the control means controls the chip clock signal generation means such that the chip clock signal fc is determined by Equation (fc=(W-b)/a, where xe2x80x9caxe2x80x9d and xe2x80x9cbxe2x80x9d, are predetermined constants). More preferably, the control means updates the target power consumption value W at initiation of the call-await time of the mobile communications device or at commencement of communication.
Preferably, the control means controls the chip clock signal generation means such that the chip clock signal fc is determined on the basis of the time T during which a receiving path of the received signal S is brought out of phase with the spreading code. At this time, the control means preferably controls the chip clock generation means such that the chip clock signal fc is determined so as to set the time T to a predetermined target value T0. Preferably, the control means controls the chip clock signal generation means such that the target value T0 assumes a value of predetermined value T1 or more and a predetermined value T2 or less (T1xe2x89xa6T0xe2x89xa6T2). Preferably, the control means calculates the predetermined value T1 according to Equation (T1=xcexc/V) on the basis of data V pertaining to the traveling speed of the mobile communications device detected by the speed detection means and a code wavelength xcexc of the chip clock signal fc. Further, the 20 control means preferably controls the chip clock signal generation means such that the chip clock signal fc is determined according to Equation (fc=kc/T, where xe2x80x9ckcxe2x80x9d is a predetermined constant).
Preferably, the control means may control the chip clock signal generation means such that the chip clock signal fc is determined on the basis of a period Td at which the computation means prepares a delay profile. At this time, the control means may calculate the period Td on the basis of the time T, according to Equation (Td=kexc3x97T, where xe2x80x9ckexe2x80x9d is a constant satisfying 0 less than ke less than 1) and control the chip clock signal generation means such that the chip clock signal fc is determined according to Equation (fc=kd/T, where xe2x80x9ckdxe2x80x9d is a predetermined constant.
Preferably, the control means can also control the chip clock signal generation means such that the chip clock signal fc is determined on the basis of a predetermined spreading rate N and a predetermined data transmission symbol rate fv. Here, the control means may control the chip clock signal generation means such that the chip clock signal fc is determined according to Equation (fc=fv/N).
Preferably, the control means can control the chip clock signal generation means such that the chip clock signal fc is determined on the basis of a communication rate M at which the communication device is charged. At this time, the control means preferably controls the chip clock signal generation means such that the chip clock signal fc is determined according to Equation (fc=km/M-xcex1, where xe2x80x9ckmxe2x80x9d is a predetermined value satisfying 0 less than km, and xcex1 is a predetermined value satisfying 0xe2x89xa6xcex1).
Preferably, the control means calculates a synchronous sample rate fs of the computation means from the amount of memory Me required for storing the delay profile prepared by the computation means and controls the chip clock signal generation means such that the chip clock signal fc is determined on the basis of the synchronous sample rate fs. Further, the control means preferably calculates a synchronous sample rate fs of the computation means from the amount of computation Mi required for the computation means to compute the delay profile and controls the chip clock signal generation means such that the chip clock signal fc is determined on the basis of the synchronous sample rate fs.
Preferably, the control means selects the chip clock signal fc generated by the chip clock generation means from the group comprising: a value calculated from the target power consumption value W of the mobile communications device, a value calculated on the basis of the time T during which the receiving path of the received signal S is brought out of phase with the spreading code, a value calculated on the basis of the predetermined spreading rate N and the predetermined data transmission symbol rate fv, a value calculated on the basis of the period Td during which the computation means prepares a delay profile, a value calculated from a communications rate M, a value calculated on the basis of the amount of memory Me required for storing the delay profile prepared by the computation means, and a value calculated on the basis of the amount of computation Mi required for the computation means to calculate a delay profile. At this time, according to the state of communication, the control means desirably selects the value of the chip clock signal fc so as to reduce the amount of hardware resources of the mobile communications device required, the communication rate M, or the amount of radio resources occupied. Preferably, the mobile communications device further comprises a user interface for enabling entry of data or indication of information and wherein the control means calculates, on the basis of the value of the selected chip clock signal fc, the target power consumption value W, the time T, the spreading rate N, the data transmission symbol rate fv, the period Td, the communication rate M, the amount of memory used Me, and the amount of computation required Mi, controls the user interface such that the user interface displays these values, and re-calculates these values in a case where a modified value of the chip clock signal fc is entered by way of the user interface.
To solve the drawbacks, the present invention also provides a mobile communications device having control means for controlling communication and communications means for establishing data communication with respect to a cell station, wherein the communications means comprises: first chip clock signal generation means for generating a first chip clock signal Fc; transceiver means for effecting transmission of information; and sync-detection-and-maintaining means for preparing a delay profile, and the sync-detection-and-maintaining means comprises: despreading code generation means for generating a first despreading code C on the basis of the first chip clock signal Fc output from the first chip clock signal generation means and despreading information output from the control means; chip clock signal generation means for generating a chip clock signal fc on the basis of the first chip clock signal Fc output from the first chip clock signal generation means; operation clock signal generation means for generating an operation clock signal fs on the basis of the clock rate value output from the control means; latch means for generating a second despreading code Cxe2x80x2 by means of latching the first despreading code C; a parallel correlator which extracts a correlation output P from a signal S received by the transceiver means on the basis of the second despreading code Cxe2x80x2 and the operation clock signal fs; and computation means for preparing a delay profile on the basis of the correlation output P.
Preferably, the latch means samples data, on the basis of the chip clock signal fc, data are sampled every xe2x80x9cnxe2x80x9d bits with respect to the first despreading code C, and makes values of the thus-sampled bits continuous to a length corresponding to xe2x80x9cnxe2x80x9d bits, to thereby produce the second despreading code Cxe2x80x2. Further, the latch means can divide, on the basis of the chip clock signal fc, the first despreading code C into xe2x80x9cmxe2x80x9d, through xe2x80x9cnxe2x80x9d bit blocks, sample arbitrary bits which are present in the blocks, and make the thus-sampled bit data sets continuous to a length corresponding to the number of bits of the blocks of interest, to thereby produce the second despreading code Cxe2x80x2. Further, the latch means preferably may divide the first despreading code C at bit positions corresponding to integral multiples of constant R, which is a real number of one or more. At this time, the latch means preferably sets the constant R to be an integer by means of rounding down, up, or off the fractional portion of integral multiples of constant R, thus determining the bit positions through use of the integer.
Preferably, the latch means divides, on the basis of the chip block signal fc, the first despreading code C into blocks of integral xe2x80x9cnxe2x80x9d bits, samples arbitrary bits which are present in the blocks, and makes the thus-sampled bit data continuous to a length corresponding to the number of bits provided in the blocks of interest, to thereby produce the second despreading code Cxe2x80x2. Alternatively, the latch means may sample, on the basis of the chip clock signal fc, bits which are present at predetermined locations within a block of the first despreading code C and make the thus-sampled bits continuous to a length corresponding to the number of bits provided in the blocks of interest, to thereby produce the second despreading code Cxe2x80x2. At this time, the latch means preferably takes bits located at the predetermined positions as leading bits of the respective blocks.
To solve the drawbacks, the present invention also provides a mobile communications device having control means for controlling communication and communications means for establishing data communication with respect to a cell station, wherein the communications means comprises: first chip clock signal generation means for generating a first chip clock signal Fc; transceiver means for effecting transmission of information; and sync-detection-and-maintaining means for preparing a delay profile, and the sync-detection-and-maintaining means comprises: despreading code generation means for generating a first despreading code C on the basis of the first chip clock signal Fc output from the first chip clock signal generation means and despreading information output from the control means; chip clock signal generation means for generating a chip clock signal fc on the basis of the first chip clock signal Fc output from the first chip clock signal generation means; operation clock signal generation means for generating an operation clock signal fs on the basis of the clock rate value output from the control means; latch means for generating a latched received signal Sxe2x80x2 by means of latching, in accordance with the chip clock signal fc, the signal S received by the transceiver means; a parallel correlator which extracts a correlation output P from a signal S received by the transceiver means on the basis of the first despreading code C and the operation clock signal fs; and computation means for preparing a delay profile on the basis of the correlation output P.
Preferably, the latch means latches, on the basis of the chip clock signal fc, a code sequence D of the received signal S, to thereby produce a code sequence Dxe2x80x2, whereby a latched received signal Sxe2x80x2 having a code sequence Dxe2x80x2 is produced. At this time, the latch means preferably latches, on the basis of the chip clock signal fc, data every xe2x80x9cnxe2x80x9d bits with respect to the code sequence D and makes values of the thus-sampled bits continuous to a length corresponding to xe2x80x9cnxe2x80x9d bits, to thereby produce the code sequence Dxe2x80x2. Alternatively, the latch means preferably divides, on the basis of the chip clock signal fc, the code sequence D into xe2x80x9cmxe2x80x9d through xe2x80x9cnxe2x80x9d bit blocks, samples arbitrary bits which are present in the blocks, and makes the thus-sampled bits continuous to a length corresponding to the number of bits of the blocks of interest, to thereby produce the code sequence Dxe2x80x2. Further, the latch means preferably divides the code sequence D into blocks at bit positions corresponding to integral multiples of constant R, which is a real number of one or more. At this time, the latch means preferably sets the constant R to be an integer by means of rounding down, up, or off the fractional portion of integral multiples of constant R, and determines the bit positions through use of the integer.
Preferably, the latch means can divide, on the basis of the chip block signal fc, the code sequence D into blocks of integral xe2x80x9cnxe2x80x9d bits, sample arbitrary bits which are present in the blocks, and make the thus-sampled bit data continuous to a length corresponding to the number of bits provided in the blocks of interest, to thereby produce the code sequence Dxe2x80x2. Alternatively, the latch means can also sample, on the basis of the chip clock signal fc, bits which are present at predetermined locations within a block of the code sequence D and makes the thus-sampled bits continuous to a length corresponding to the number of bits provided in the blocks of interest, to thereby produce the code sequence Dxe2x80x2. At this time, the latch means preferably takes bits located at the predetermined positions as leading bits of the respective blocks.
Preferably, the control means spreads transmission data on the basis of the first chip clock signal Fc and the first despreading code C and despreads received data on the basis of the chip clock signal Fc and the first despreading code C.
Preferably, the control means can determine the target communication quality value Q0. Further, the mobile communications device preferably comprises a user interface for enabling entry of data or indication of information, and the control means may determine the target communication quality value Q0 on the basis of a signal entered by way of the user interface.
Preferably, the control means can calculate the quality Q of current communication and monitor whether or not the communication quality Q is deteriorated when compared with the target communication quality value Q0.
Preferably, the control means can determine a data transmission symbol rate fv for communication. Further, the control means can determine the communication data transmission symbol rate fv the basis of a signal entered by way of the user interface. Preferably, the control means determines the data transmission symbol rate fv on the basis of the spreading rate N and the chip rate fc and can calculate the data transmission symbol rate fv according to Equation (fv=fc/N) and determines the thus-calculated data transmission symbol rate fv.
To solve the drawbacks, the present invention further provides a communications system including a plurality of mobile communications devices for transmitting and receiving information by means of communication, a cell station which receives communication requests output from the plurality of mobile communications devices and controls communication between the mobile communications devices, and a control station for calculating a communication rate Mat which the plurality of mobile communications devices are charged, wherein the mobile communications devices correspond to the mobile communications device as defined in any one of claims 1 through 69, and the cell station adopts a different chip clock signal fc for the communication established by each of the mobile communications devices.
Preferably, the mobile communications device sends the cell station a data transmission symbol rate fv, a target communication quality value Q0, and a chip rate fc; the cell station has communication initiation-and-determination means for determining whether or not communication is available, on the basis of the data transmission symbol rate fv, the target communication quality value Q0, and the chip rate fc output from the mobile communications device; and the control station has communication rate calculation means for calculating a communication rate M on the basis of the chip rate fc output from the cell station.
Preferably, the communication rate calculation means of the control station calculates a communication rate on the basis of the chip rate fc, according to Equation (M=km/fc+xcex1 or M=kmxc3x97fc+xcex1, where xe2x80x9ckmxe2x80x9d, is a constant satisfying 0 less than km, and xcex1 is a constant satisfying 0xe2x89xa6xcex1).
Preferably, the communication initiation-and-determination means of the cell station can determine, with respect to the mobile communications device, a data transmission symbol rate fvxe2x80x2, a target communication quality value Q0xe2x80x2, and a chip rate fcxe2x80x2, which enable approval of communication.
Preferably, in a case where the quality Q of communication established with respect to the mobile communication device is inferior to the target communication quality value Q0, the cell station has transmission power calculation means which updates power Pw used for transmitting a signal to the mobile communications device by means of changing the chip rate fc, to thereby cause the communication quality Q to satisfy the target communication quality value Q0. At this time, the transmission power calculation means can calculate a communication rate on the basis of the chip rate fc, according to Equation (Pw=kp/fc+xcex2, where xe2x80x9ckpxe2x80x9d is a constant satisfying 0 less than km, and xcex2 is a constant).
Preferably, the communications system corresponds to a spread-spectrum communications system, and the spread-spectrum communications system desirably corresponds to a code division multiple access (CDMA) communications system.
To solve the drawbacks, the present invention also provides a communications method to be performed by: a plurality of mobile communications devices for transmitting and receiving information by means of communication, a cell station which receives communication requests output from the plurality of mobile communications devices and controls communication between the mobile communications devices, and a control station for calculating a communication rate M at which the plurality of mobile communications devices are charged, the method comprising the steps of: causing a mobile communications device for issuing a communication request to determine and send, to the cell station, a data transmission symbol rate fv, a target communication quality value Q0, and a chip rate fc; causing the cell station to determine whether or not communication is available, on the basis of the data transmission symbol rate fv, the target communication quality value Q0, and the chip rate fc; causing the control station to determine the communication rate M on the basis of the chip rate fc in a case where communication is available; causing the mobile communications device to determine whether to start communication, on the basis of the communication rate, as well as an optimum chip rate fc; causing the mobile communications device to monitor communication quality Q; and determining a plurality of communication parameters on the basis of the chip rate fc, to thereby maintain communication through use of the plurality of communication parameters.
Preferably, in a case where it is determined that communication requiring the data transmission symbol rate fv, the target communication quality value Q0, and the chip rate fc is disapproved, the step of determining whether or not communication is available involves repetition of steps of: sending, to the mobile communications device, a data transmission symbol rate fvxe2x80x2, a target communication quality value Q0xe2x80x2, and a chip rate fcxe2x80x2, which enable approval of communication; and causing the mobile communications device to newly determine and send, to the cell station, another data transmission symbol rate fv, another target communication quality value Q0, and another chip rate fc, on the basis of the mobile communications device a data transmission symbol rate fvxe2x80x2, a target communication quality value Q0xe2x80x2, and a chip rate fcxe2x80x2.
Further, in a case where the communication quality Q is inferior to the target communication quality value Q0, the step of monitoring the communication quality Q preferably involves an operation for sending a request to the cell station for increasing transmission power Pw and an operation for causing the cell station to update the chip rate fc in response to the request, to thereby increase the transmission power Pw, and the step of maintaining communication preferably involves an operation for updating of the plurality of communication parameters on the basis of the thus-updated chip rate fc and an operation for maintaining communication through use of the communication parameters.
In the mobile communications device, the communications system, and the communications method according to the present invention, the mobile communications device can set the spreading (despreading) chip rate fc. Therefore, the spreading (despreading) chip rate fc is set to be low, thereby enabling broadening of the width of a phase during which a correlation output P is produced. The unit of phase shift required for executing a sync-determination-and-maintaining operation (i.e., preparation of a delay profile) can be lengthened, so that the number of mathematical calculations (i.e., the amount of computation) and the amount of information (i.e., storage capacity) can be diminished.
At the time of the sync-determination-and-maintaining operation, a parallel correlator called a matched filter is in operation, and the power consumed by the matched filter accounts for the majority of the power consumption of a mobile station(i.e., a mobile communications device), such as a cellular phone. Therefore, so long as the width of a phase during which a correlation output P is produced is broadened by means of reducing the spreading (despreading) chip rate fc, a correlation can be detected even if the unit of phase shift corresponding to the period of an operation clock of the matched filter is broadened. Thus, the power consumed by the matched filter; that is, the power consumed by the mobile communications device, can be diminished.
Since a time interval during which a received signal is out of phase with the spreading code becomes longer by means of reducing the spreading (despreading) chip rate fc, the time interval during which a sync-determination-and-maintaining operation is performed can be made longer, thereby reducing the number of times the matched filter is activated per unit time.
The transmission power becomes higher in order to prevent deterioration of communication quality, which would otherwise be caused when the spreading (despreading) chip rate fc is reduced, thereby occupying a large amount of radio resources and reducing the number of available calls. For this reason, the amount of a bill is determined on the basis of the spreading (despreading) chip rate fc. As a result, a payment higher than an ordinary rate is charged to a person who desires enhanced convenience, thereby redressing an inequity between users and flexible, equitable, and rational management of a communications system.
In the mobile communications device, the communications system, and the communications method according to the present invention, the period Td at which a delay profile is prepared can be controlled in accordance with the period T at which synchronization between chips is lost and which corresponds to the traveling speed of the mobile communications device, thereby enabling preparation of a delay profile at an optimum period Td. Therefore, the number of mathematical calculations (i.e., the amount of computation) and the amount of information (i.e., storage capacity) can be reduced. Particularly, when the mobile communications device is stationary or travels at low speed, an operation for preparing an undesired delay profile can be omitted.
Further, the range of phase Tp during which a correlation output P is produced can be controlled by means of setting the spreading (despreading) chip rate fc. Therefore, when the period T during which synchronization between chips is lost is short, the spreading (despreading) chip rate fc is set to be low, thereby prolonging the period Td at which a delay profile is prepared. For this reason, the number of times a delay profile is prepared per unit time can be diminished, thus alleviating processing load. Further, the number of times a matched filter is operated per unit time is also reduced, thus diminishing power consumption.
With regard to the spreading (despreading) chip rate fc and the period Td at which a delay profile is prepared, the period Td can be determined on the basis of the spreading (despreading) chip rate fc, or the spreading (despreading) fc can be determined on the basis of the period Td. Consequently, communication can be established according to the state of the user, thus enabling flexible, equitable, and rational management of a communications system.
Even in the case of spread-spectrum communication in which the spreading (despreading) chip rate Fc of a mobile station is fixed, a mobile communications device (i.e., the mobile station) can select a different second receiving chip rate fc by means of a latch section provided in the sync-detection-and-maintaining section, thus achieving the advantages as discussed above.